The present invention relates to a performance evaluation method of an MR (Magneto-Resistance) head and a magnetic disk device wherein the method is applied.
Recently, improvement of read/write resolution of a magnetic head is strongly required along with miniaturization and recording capacity enlargement of magnetic recording devices such as a hard disk unit.
In order to reply to this requirement, the MR sensor, which senses resistance variation caused by a magnetic field, has become widely used as a reading element of the magnetic head. This is because the resistance variation of the MR sensor depends only on the magnetic field generated by magnetization transitions recorded on the magnetic disk, being independent of relative speed of the magnetic disk to the MR sensor.
In the MR head, the resistance of the MR sensor varies as a function of magnetic field intensity. Therefore, the output signal of the MR head has a wave form which represents magnetization transitions recorded on the recording medium.
Conventionally, an isolated reproduction wave form V(t), which is obtained when a MR head reads a single magnetization transition, is treated to be represented by following equation, by normalizing it, peak value as 1. EQU V(t)=1/(1+(2t/PW50).sup.2) (2)
Here, t denotes time difference from a timing which gives the peak value of V(t), and a half-peak-width PW50 denotes a time width where V(t) shows more than 50% of the peak value.
Making use of the above equation (2), the BER (Bit-Error-Rate), that is, the rate of error bits to total bits in a reproduced signal, of a magnetic disk device to be designed can be estimated with computer simulation, according to a ratio K of the half-peak-width PW50 to the minimum bit-interval BI (K=PW50/BI) and SN (Signal-Noise) ratio of the reproduced signal.
More concretely, from a reproduction output for a maximum recording density, which is obtained as a linear summation .SIGMA..sub.i V(t+.sub.i BI) of the isolated reproduction wave forms of equation (2), the BER is calculated according to the SN ratio for each ratio K, and the designing factors of the magnetic disk device are determined referring to the calculation results. The better is the smaller BER realized with the smaller SN ratio, of course.
The minimum bit-interval BI corresponds to minimum distance between two consecutive magnetization transitions, and so, the half-peak-width PW50 is usually expressed with unit of length being multiplied by relative speed of the MR head to the magnetic disk.
FIG. 9 is a graphic chart illustrating the BER relative to the SN ratio calculated for three values of the ratio K, K=2.0, 2.4 and 2.8, on condition that the reproduction signal is to be processed according o the 8/9 modulation, class IV, partial-response maximum-likelihood PR4ML) decoding method.
As can be understood from the graphic chart of FIG. 9, the ratio K should be small for realizing a required BER, 1.0E-5, for example, with a small SN ratio. The small value of the ratio K means a narrow half-peak-width PW50 for a certain high recording density.
In case the half-peak-width PW50 is not sufficiently narrow, the output amplitude of the reproduced signal is decreased when reading magnetization transitions recorded close to each other, because of interference among the isolated reproduction wave forms, even if a sufficient output amplitude is obtained when reading magnetization transitions recorded separately.
Therefore, the half-peak-width PW50 defined by equation (2) has been used conventionally as the performance evaluation parameter of the MR head for designing magnetic disk devices, and a narrower half-peak-width PW50 has been sought for realizing higher recording density.
However, there are found problems in evaluating performance of the MR heads only according to the half-peak-width PW50.
The first problem is that the required recording density cannot be obtained in some products of the magnetic disk device even where the half-peak-width PW50 of their MR heads are within the designed norm, resulting in a low yield rate and a low productivity. This is because the isolated reproduction wave form representing resolution performance differs a little for each MR head even when the half-peak-width PW50 is the same, and this difference considerably affects the resolution performance of each MR head in case the recording density is high.
The second problem is that the resolution performance of the MR head degrades according to usage in some products of the magnetic disk device, resulting in low reliability of the magnetic disk device. This is because the isolated reproduction wave form may be changed even when the half-peal-width PW50 remains to be the same, along with change of the magneto-resistance characteristic of the MR sensor which may be caused as a result of the recording effect of the MR sensor, for example.